The term "sticky polymer" is defined as a polymer, which, although particulate at temperatures below the sticking temperature, agglomerates at temperatures above the sticking temperature. The term "sticking temperature", which, in the context of this specification, concerns the sticking temperature of particles of polymer in a fluidized bed, is defined as the temperature at which fluidization ceases due to the agglomeration of particles in the bed. The agglomeration may be spontaneous or occur on short periods of settling.
A polymer may be inherently sticky due to its chemical or mechanical properties or pass through a sticky phase during the production cycle. Sticky polymers are also referred to as non-free flowing polymers because of their tendency to compact into aggregates of much large size than the original particles and not flow out of the relatively small openings in the bottom of product discharge tanks or purge bins. Polymers of this type show acceptable fluidity in a gas phase fluidized bed reactor; however, once motion ceases, the additional mechanical force provided by the fluidizing gas passing through the distributor plate is insufficient to break up the aggregates which form and the bed will not refluidize. These polymers are classified as those, which have a minimum bin opening for free flow at zero storage time of up to two feet and a minimum bin opening for free flow at storage times of greater than five minutes of 4 to 8 feet or more.
Sticky polymers can also be defined by their flow. This is called the Flow Factor, which references the flow of all materials to that of dry sand. On a scale of 1 to 10, the Flow Factor of dry sand is 10. The Flow Factor of free flowing polymers is about 4 to 10 while the Flow Factor of non-free flowing or sticky polymers is about 1 to 3.
Because of the tendency to agglomerate, sticky polymers are difficult to produce in typical gas phase processes, which are usually carried out in fluidized beds. Both economic and safety/environmental considerations indicate, however, that fluidized bed type polymerization is preferred for the manufacture of polymers that can exist in a granular, fluidizable form.
Although polymers that are sticky can be produced in non-gas phase processes, there are certain difficulties associated with the production of such products in, for example, slurry or bulk monomer polymerization processes. In such processes, the diluent or solvent is present in the resins exiting the reaction system at a high concentration leading to severe resin purging problems, particularly if the material in question is a low molecular weight resin or a very low crystallinity resin. Environmental considerations are such that the dissolved monomers and diluent must be removed from the polymer prior to its exposure to air. Safety also dictates the removal of residual hydrocarbons so that closed containers containing the polymers will not exceed safe volatiles levels in the gas head space over the resin. The safety and environmental concerns are accompanied by a definite economic factor in determining a preference for a gas phase fluid bed reaction system. The low number of moving parts and the relative lack of complexity in a basic fluidized bed process enhances the operability of the process and typically results in lower costs of production. Low costs of production are due, in part, to low volumes of recycled process streams and a high unit throughput.
Three major process types are currently used for the production of some of these sticky resins. (1) The bulk monomer slurry process is quite efficient for contacting monomers with catalyst and obtaining high productivity. Some of the disadvantages associated with this process are the relatively high pressures used; and the very high concentration of dissolved monomer in the resin exiting the reactor. This type of process is characterized by a relatively small volume main reactor coupled to extensive monomer recovery/polymer flash and recovery facilities. (2) The diluent slurry process operates in a manner similar to the bulk monomer slurry process; however, the reactor tends to be larger and of lower pressure capability due to a lower monomer concentration requiring a larger reactor volume for the same rate of polymerization. The same disadvantages of the bulk slurry process are shared by the diluent slurry process. If the polymer is permitted to dissolve in the diluent, the solution viscosity increases drastically leading to reactor fouling. Relatively large diluent/monomer recovery systems must be maintained to economically recover same from purge bins and product recovery systems. (3) The solution process allows operation at higher reaction temperatures with improved heat removal. It also allows high polymerization rates for given reactor size due to the usually positive effect of reaction temperature on the activity of the polymerization catalyst. A major disadvantage of the solution process is the typically cumbersome recovery methods required for polymer and solvent. These methods require a significant amount of equipment and produce the final polymer in pellet form, which can be difficult to purge of residual monomer and solvent to environmentally safe and acceptable levels. One additional safety factor involved in the use of a solution process is the presence of a large volume of hot solvent, frequently well above the flash point, which contains a significant amount of dissolved polymer. Spills of this type of material present significant hazards with regard to fire and personnel exposure. A further disadvantage of the solution process is that the rate and, to some extent, the operability of the system is dependent on the molecular weight and solubility of the particular product being produced. Products of higher density than desired (or designed for) will precipitate in cool spots in the lines and foul the reaction system. Products of higher molecular weight will increase solution viscosity to the point that the design capability of the circulation system will be exceeded and operations will become impractical. A major product deficiency of the solution processes is that they are inherently incapable of production of desirable high molecular weight or ultrahigh molecular weight grades of resin.
All of the above processes, although usable for the production of many different types of polymers, have deficiencies that are not present in the fluidized bed reaction system. The absence of large volumes of solvent or liquid monomer increases the safety of the system. The granular nature of the resultant polymer increases the flexibility of the system in that both granular resin and compounded resin can be delivered to the customer. The granular, porous nature of the polymer also facilitates purging of unwanted monomer to environmentally safe levels. A wide range of molecular weights can be produced in a fluidized bed, i.e., from ultrahigh molecular weights having a melt index of less than 0.001 to relatively low molecular weights having a melt index of up to 100. Melt index is measured under ASTM D-1238, Condition E, at 190.degree. C. and reported as grams per 10 minutes. The high heat removal capacity of a fluidized bed (due to the circulation of the fluidizing gas) and the ability to control reaction concentrations without the limitations imposed by the solubility of components such as hydrogen in the diluent are also desirable features of the fluidized bed process.
It is clear, then, that the production of polymer by means of a fluidized bed reaction system is advantageous. A typical system of this type is described in U.S. Pat. No. 4,482,687, which is incorporated by reference herein. Unfortunately, this system requires that the granular product be free-flowing.